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Researchers from Latvian and Estonian universities (Estonian University of Life Sciences and the University of Tartu) who have studied great tits (Parus major) in a managed forest and in an old unmanaged forest have found that the biomass of insects is lower in the industrial forest and that the great tit’s young are in poorer health.
Although the survival of the young did not differ between forests, the young that fledged in the managed forest were more stressed and had shorter telomeres, suggesting a shorter lifespan. Telomeres are the parts at the top of chromosomes whose shortening is associated with age-related diseases, including cancer. It is known that telomere shortening reduces lifespan in birds, and also that telomeres shorten more rapidly under stress and when habitat quality is lower.
Over two years (2018–2019), researchers measured great tits nestlings and the abundance of insects in 40–50-year-old pine plantations and 110–160-year-old unmanaged mixed spruce-dominated forests around Kraslava in Latvia. Stress was also measured in the birds’ blood.
Stress levels affect health, development, growth and reproduction. While short-term disruption causes transient stress, long-term disruption results in chronic stress, which weakens the immune system and increases the risk of disease.
To cope with this, you need to expend extra energy, i.e. eat more. The state of the immune system can be measured by counting white blood cells: a high number of pathogen-fighting lymphocytes indicates that something is wrong in the body. It is known that the ratio of heterophils (also a type of white blood cell) to lymphocytes (H/L) reflects immunological stress, but also changes in the environment, including those related to food shortages. The study showed that the stress indicator, H/L, was significantly lower in young birds reared in the old unmanaged forest: 0.2 in old forest vs 0.8 in managed forest.
Although the size of the clutch and the number of young that left the nest did not depend on the age of the forest or management, the insects that great tits and other woodland birds feed their young were significantly more abundant in the old forest. The telomeres of the young were also significantly longer in forests over 100 years old. As the habitat deteriorates, the bird’s stress hormone levels rise, which in turn leads to a weakened immune system and reduces the likelihood of survival.
Due to intensive forestry, old forests, including old coniferous forests in northern Europe, are disappearing. Clear-cutting, the creation of patches of young forest, accelerating fragmentation of forest areas, and the removal of undergrowth are all consequences of forest management that have reduced the quality of forest habitats and are associated with the decline of populations of several common forest species.
The authors conclude that although the great tit is happy to nest in a nest box and can seemingly breed everywhere anything, the young are more stressed in young managed forests and have significantly less suitable food than in old unmanaged forests.
Also, the great tit may be in an ecological trap in a managed forest, which cannot be detected by counting the birds and for which reason the authors of the study do not recommend placing nest boxes in a young managed forest. Although it has been suggested that birds may adapt to intensive forestry, the data on Latvian great tits do not suggest this. However, the study suggests that intensive forestry can turn managed forest habitat into an ecological trap.
This article is written by Marko Mägi. It was originally published on the “Birdwatcher blog”.
If this chirpy read has got you flocking for more, don’t just wing it! Fly over to our next article and peck up more about how Avian flu can also wreak havoc on cormorant colonies!
